fix(langchain): handle prompt_tokens_details as dict in _parse_usage_model

When LiteLLM proxy or OpenAI returns prompt_tokens_details as a dict
(e.g. {"cached_tokens": 12000}), _parse_usage_model only handled the
Vertex AI list format and silently dropped the dict via the
isinstance(v, int) filter on line 1318.

This caused cached token counts to be lost and input costs to be
inflated in Langfuse, since prompt_tokens was never adjusted for
cache hits.

Add dict handling for prompt_tokens_details mirroring the existing
input_token_details pattern: flatten keys as input_{key}, subtract
from input total. Existing Vertex AI list handling is preserved
via elif.

Closes langfuse/langfuse#13024

Author: tanyelai

langfuse/langchain/CallbackHandler.py

@@ -1256,8 +1256,25 @@ def _parse_usage_model(usage: Union[pydantic.BaseModel, dict]) -> Any:
                 if "output" in usage_model:
                     usage_model["output"] = max(0, usage_model["output"] - value)
 
-        # Vertex AI
+        # OpenAI / LiteLLM — prompt_tokens_details as dict
+        # e.g. {"cached_tokens": 12000}
         if "prompt_tokens_details" in usage_model and isinstance(
+            usage_model["prompt_tokens_details"], dict
+        ):
+            prompt_tokens_details = usage_model.pop("prompt_tokens_details")
+
+            for key, value in prompt_tokens_details.items():
+                if not isinstance(value, int):
+                    continue
+
+                usage_model[f"input_{key}"] = value
+
+                if "input" in usage_model:
+                    usage_model["input"] = max(0, usage_model["input"] - value)
+
+        # Vertex AI — prompt_tokens_details as list
+        # e.g. [{"modality": "text", "token_count": N}]
+        elif "prompt_tokens_details" in usage_model and isinstance(
             usage_model["prompt_tokens_details"], list
         ):
             prompt_tokens_details = usage_model.pop("prompt_tokens_details")

tests/test_parse_usage_model.py

@@ -16,6 +16,68 @@ def test_standard_tier_input_token_details():
     assert result["total"] == 14
 
 
+def test_prompt_tokens_details_dict_cached_tokens():
+    """OpenAI/LiteLLM: prompt_tokens_details as dict with cached_tokens."""
+    usage = {
+        "prompt_tokens": 15000,
+        "completion_tokens": 500,
+        "total_tokens": 15500,
+        "prompt_tokens_details": {"cached_tokens": 12000},
+    }
+    result = _parse_usage_model(usage)
+    assert result["input"] == 3000  # 15000 - 12000
+    assert result["output"] == 500
+    assert result["total"] == 15500
+    assert result["input_cached_tokens"] == 12000
+
+
+def test_prompt_tokens_details_dict_with_cache_creation():
+    """OpenAI/LiteLLM: prompt_tokens_details dict + top-level cache_creation."""
+    usage = {
+        "prompt_tokens": 15000,
+        "completion_tokens": 500,
+        "total_tokens": 15500,
+        "prompt_tokens_details": {"cached_tokens": 12000},
+        "cache_creation_input_tokens": 3000,
+    }
+    result = _parse_usage_model(usage)
+    assert result["input"] == 3000  # 15000 - 12000 (cached_tokens only subtracted here)
+    assert result["input_cached_tokens"] == 12000
+    assert result["cache_creation_input_tokens"] == 3000
+
+
+def test_prompt_tokens_details_list_vertex_ai():
+    """Vertex AI: prompt_tokens_details as list — existing behavior preserved."""
+    usage = {
+        "prompt_token_count": 1000,
+        "candidates_token_count": 200,
+        "total_token_count": 1200,
+        "prompt_tokens_details": [
+            {"modality": "text", "token_count": 800},
+            {"modality": "image", "token_count": 200},
+        ],
+    }
+    result = _parse_usage_model(usage)
+    assert result["input"] == 0  # 1000 - 800 - 200
+    assert result["output"] == 200
+    assert result["total"] == 1200
+    assert result["input_modality_text"] == 800
+    assert result["input_modality_image"] == 200
+
+
+def test_prompt_tokens_details_dict_empty():
+    """Empty dict prompt_tokens_details — no crash, input unchanged."""
+    usage = {
+        "prompt_tokens": 5000,
+        "completion_tokens": 100,
+        "total_tokens": 5100,
+        "prompt_tokens_details": {},
+    }
+    result = _parse_usage_model(usage)
+    assert result["input"] == 5000
+    assert result["output"] == 100
+
+
 def test_priority_tier_not_subtracted():
     """Priority tier: 'priority' and 'priority_*' keys must NOT be subtracted."""
     usage = {